1. What are ecoregions?
Ecoregions are the actual scale on which CarbonTracker performs its
optimization over the land. Ecoregions are meant to represent large
expanses of land within a given continent having similar ecosystem
types, and are used to divide continents into smaller pieces for
analysis. The ecosystem types use in CarbonTracker are derived from
the Olson [1992] vegetation classification (Table 1,
Figure 1).

We define an ecoregion as an ecosystem type within a given
Transcom land region. There are 11 such Transcom land regions (Figure
2), so there are 11*19 = 209 possible ecoregions. However, not all
ecosystem types are present in all Transcom regions, and the actual
number of land ecoregions ends up being 126.

Note on "Semitundra": this is a potentially misleading shorthand
abbreviation for a collection of ecosystems comprising semi-desert,
shrubs, steppe, and polar+alpine tundra. The "Semitundra" zones
appearing in northern Africa where one expects to find the Sahara
desert are not, of course, tundra environments. They are instead
semi-desert zones.

Figure 1. Global distribution of Olson ecosystem types.

Table 1. Ecosystem areas over the two Transcom regions covering North America.

Ecosystem Type

North American Boreal

North American Temperate

Area (km2)

Percentage

Area (km2)

Percentage

Conifer Forest

2315376

22.9%

1607291

14.0%

Broadleaf Forest

-

-

269838

2.4%

Mixed Forest

592291

5.9%

930813

8.1%

Grass/Shrub

53082

0.5%

2515582

21.9%

Tropical Forest

-

-

58401

0.5%

Scrub/Woods

-

-

416520

3.6%

Semitundra

3396292

33.6%

866468

7.6%

Fields/Woods/Savanna

29243

0.3%

1020939

8.9%

Northern Taiga

1658773

16.4%

-

-

Forest/Field

61882

0.6%

1243174

10.8%

Wetland

322485

3.2%

66968

0.6%

Deserts

-

-

21934

0.2%

Shrub/Tree/Suc

-

-

11339

0.1%

Crops

-

-

1969912

17.2%

Conifer Snowy/Coastal

41440

0.4%

73437

0.6%

Wooded tundra

360388

3.6%

6643

0.1%

Mangrove

-

-

-

-

Non-optimized areas

-

-

-

-

Water

1269485

12.6%

384728

3.4%

Total

10100736

100.0%

11463986

100.0%

2. Why use
ecoregions?
A fundamental challenge to atmospheric inversions like CarbonTracker
is that there are not enough observations to directly constrain fluxes
at all times and in all places. It is therefore necessary to find a
way to reduce the number of unknowns being estimated. Strategies to
reduce the number of unknowns in problems like this one generally
impose information from external sources. In CarbonTracker, we reduce
the problem size both by estimating fluxes at the ecoregion scale, and
by using a terrestrial biological model to give a first guess flux
from the ecoregion. The model is also used to give the spatial and
temporal distribution of CO2 flux within a
region and week.

2. Ecosystems within Transcom regions
Each Transcom land region (Figure 2) can contain up to 19 ecoregions.